The primary aim of this research is to understand how gonadal steroid hormones shape the nervous system and behavior during puberty in male hamsters. While the pubertal rise in testosterone (T) is often associated with the activation of male reproductive behavior, these T secretions also organize (i.e., permanently alter) developing neural circuits to enhance activational responses to T in adulthood. Exp. 1 will test the hypothesis that puberty is a sensitive period for organizational actions of T on masculine reproductive behavior. Specific Aim 2 will determine whether T during puberty organizes masculine reproductive behavior by altering medial preoptic area dopaminergic responses to female sensory stimuli. These studies have implications for human conditions such as anabolic steroid abuse, eating disorders, and disease, all of which shift the window of brain exposure to steroid hormones outside of the normal time of puberty. [unreadable] [unreadable]